Audio systems are increasingly relying upon Class D switching output amplifiers for their audio power amplification needs. Relative to linear amplifiers, Class D audio amplifiers are much more efficient while not giving up too much in terms of linear performance (e.g., showing low distortion and high dynamic range). There are several types of Class D amplifiers, in relation to the manner in which their switching power stages are controlled. In one type, a periodic ramp signal having a pulse width modulation (PWM) carrier frequency (that is typically much higher than the audio range) is generated and compared to the input audio signal. The output of the comparison is a variable pulse width square wave signal whose pulse width becomes longer when the input audio amplitude is larger, shorter when the input audio amplitude is smaller. This output signal (which may be considered to have a fixed switching frequency) is then applied to a switching type power stage, and the output of the power stage is low-pass filtered to remove the PWM carrier, yielding an amplified audio signal. Another type of Class D amplifier is a self-oscillating type in which the variable pulse width square wave signal is produced by a self-oscillating circuit receiving the input audio signal. The switching frequency in that case is not fixed, but rather varies depending on the audio input level. An external oscillator may be added that injects charge into the amplifier, so as to synchronize or lock the switching frequency to a desirable value. The external oscillator may also be made variable, e.g. as a spread spectrum type that shifts the switching frequency around randomly to improve immunity against electromagnetic interference.